Corneal contact lenses are designed to have physical properties and shapes compatible with the physical and physiological characteristics of the user's eye and to have optical properties providing the necessary visual correction. While most basic types of contact lenses are constituted by spherical surfaces there exists a significant number of users who exhibit toric cornea, a characteristic physical configuration of the corneal curvature. Corneal toricity is recognized as the prime contributor to an astigmatic refractive error but, when the toricity is not too great, it may be present without producing an astigmatism. The astigmatic refractive error is one property of the light rays that have passed through the eye's various refractive interfaces to form an image upon the retina. Accordingly, lens types more complex than spherical surfaces, which are constituted by one or two toric surfaces are often required to achieve an optimum physical fit, an optimum correction of the refractive error, or both. It is recognized that such lenses are more costly, more difficult to manufacture accurately, and more difficult to fit effectively than are the spherical surface-types of lenses. See generally the discussion in Mandell, Contact Lens Practice, Second Edition, pp. 565-602, the disclosure of which is hereby incorporated by reference.
Toric corneal contact lenses were first introduced by Stimpson in 1950 and the toric base corneal contact lens and its use were described in 1952 and 1953 by Schapero and Baglien. Today the toric back surface corneal contact lens is used to obtain an optimum lens-cornea bearing relationship when the toricity of the cornea precludes the use of a spherical back surface lens. Thus the principal use is to fit a toric cornea and not to correct an astigmatic refractive error or residual astigmatism; see Mandell, above.
Prior art procedures for the manufacture of toric contact lenses are time-consuming and fairly expensive involving a considerable amount of hand finishing. Moreover, the apparatus used to fabricate such lenses was often elaborate and difficult to achieve reproducible results. Illustrative patents include U.S. Pat. Nos. 3,079,737; 3,344,692; 3,482,906; 3,535,825; 3,623,800 and 3,950,082.
Spherical contact lenses may be prepared by a casting method in order to reduce or eliminate costly machining and polishing of the lens as described in U.S. Pat. No. 4,121,896, issued Oct. 24, 1978 in the name of T. H. Shepherd. According to the process described in this United States patent male and female mold members are prepared from a thermoplastic composition by injection molding. The plastic mold parts are then used to mold spherical corneal contact lenses, one of the molding surfaces being provided with an integral, flexible circumferential ring which defines the periphery of the resulting lens. The material which will constitute the lens is charged between the two mold parts, the mold parts are contacted and during molding, the molding material contracts causing the two mold portions to move towards each other. Once the molding is completed the mold parts are separated and the resulting contact lens is removed in a condition ready for use except for cleaning and polishing of the edges.
It is an object of the present invention to provide a molding apparatus and a method using the apparatus for fabricating toric contact lenses at relatively low cost without the need for extensive and expensive manual finishing operations.
Another object of the present invention is to produce a toric-shaped contact lens, which may be either hard or flexible, in a closed mold system which compensates for shrinkage that occurs on polymerization.
It is also an object of this invention to provide a method for producing cast toric lenses in a closed mold system, such lenses having the desirable optical and fitting characteristics of machined and polished lenses.
These and other objects will be apparent from the following description.